1. Field of the Invention
The present invention generally relates to an optical pickup apparatus used for recording and reproducing record information by emitting a light beam from a light source onto an information record medium, and more particularly to an optical pickup apparatus for performing a power control of the light beam by detecting the light amount of the light beam emitted from the light source by means of a front monitor method.
2. Description of the Related Art
In a semiconductor laser as a light source of the optical pickup, since the emission power is changed due to a temperature change and aging, a stabilization of the power level of the light beam emitted onto the information record medium such as an optical disc is performed by conducting a power control by means of the APC (Automatic Power Control). As a representative method of this APC, there is a rear monitor method and a front monitor method.
Since, in the rear monitor method, the light beam emitted from the end surface of the semiconductor laser on the opposite side of the emission surface, there is a problem in a detection accuracy etc., resulting in that the front monitor is in fact employed. In the front monitor method, one portion of the light beam emitted from the semiconductor laser is monitored and is fed-back to a driving circuit of the semiconductor laser, so as to keep constant the power of the light beam.
For example, in the front monitor method, the light beam emitted from a semiconductor laser is inputted to a beam splitter. This beam splitter has a half reflection film, which transmits about 90% of the light beam emitted from the semiconductor laser and reflects the rest of it i.e., about 10% of it. By the action of this half reflection film of the beam splitter, about 90% of the light beam inputted to the beam splitter is transmitted therethrough to an optical disc through some more optical elements such as an objective lens.
On the other hand, about 10% of the light beam, which is reflected by the half reflection film of the beam splitter, is guided to a front monitor detector. The front monitor detector outputs an electric signal corresponding to a light amount of the emitted light beam. This electric signal is supplied to an APC circuit including a laser control circuit for performing a power control of the semiconductor laser. The APC circuit obtains a driving signal optimum for driving the semiconductor laser in accordance with the electric signal from the front monitor detector. In this manner, the output power of the semiconductor laser is controlled by the APC circuit on the basis of the electric signal outputted from the front monitor detector.
However, in the above mentioned optical pickup apparatus employing the front monitor method, since about 10% of the light beam is reflected by the half reflection film of the beam splitter and is guided to the front monitor detector, the power of the light beam emitted onto the optical disc is correspondingly reduced, resulting in that the usage efficiency of the light beam is reduced, which is a problem.
Further, since the property of the half reflection film of the beam splitter such as the reflectivity, the transmissibility or the like has a dispersion of about .+-.5%, it is necessary to suppose the change in the reflection of the light beam within a wide range of 5% to 15%, so that the design and the adjustment become difficult to perform. Furthermore, the property such as the reflectivity, the transmissibility or the like of the half reflection film of the beam splitter is changed in accordance with the environmental temperature, the humidity and the like, it becomes a problem that the reliability of the power control by means of the APC is certainly degraded.